An image to be printed or displayed is generally represented by an array of pixel elements, sometimes referred to as a bit-mapped array, which function as an array of control codes which control the operation of printing-elements or display-elements which, in turn, produce the printed or displayed image. The bit-mapped array is generally constructed based on an input, continuous tone, image using a halftoning process which may incorporate hardware or software or a combination of both. Each pixel element in the bit-mapped array is represented by data bits which define attributes of the printed or displayed pixel, such as pixel color and gray level. A monochrome image is generally defined by a single array of pixel elements, wherein the data bits of each pixel element represent the gray level of the printed or displayed pixel. A color image is generally defined by a plurality of arrays of pixels, wherein each array corresponds to a base-color, such as cyan, magenta, yellow and black.
Halftoning methods and apparatus are described, for example in U.S. Pat. No. 4,499,489, to Gall et al., and in U.S. Pat. No. 4,084,183, to Keller et al.
The resolution of an image is generally determined by the density of pixels used to represent the image. The range of available gray levels per pixel is generally determined by the number of data bits used for representing each pixel. For example, a system using 8 bits per pixel yields a range of 256 gray levels per pixel, in monochrome printing, or 64 gray levels per pixel in 4-color printing.
Due to limitations in processing speed and data storage capacity, the resolution and gray level separation capability of existing imaging systems is limited, particularly when images are processed "on the fly" in high speed printing systems. Therefore, in existing systems, image resolution must often be compromised in order to obtain a wider gray level range or vice versa.